This invention relates generally to plastic extrusion systems and, more particularly, to reducing gear marks and ripples on plastic sheet extruded with such systems.
In at least one known plastic extrusion system, plastic resin is extruded through a screw extruder to form a plastic sheet. The extruded plastic sheet is pulled by a pull roll and drawn under pressure between a first nip roll and a second nip roll. The nip rolls and the pull roll, also referred to herein as the active rolls, are independently driven by separate motors (DC electric motors are commonly used) via two-stage worm gear transmissions.
Roll speed fluctuations between the rolls typically cause ripples, or gear marks, to appear on an extruded plastic sheet. These ripples and gear marks diminish the optical quality of the sheet. Other factors that may cause ripples in the sheet include differences in the nip force between the rolls, the bearings, and type and the amount of lubricants. Ripples also are caused by differences in sheet flow rates at the nip rolls and the pull roll, differences in roll speeds, and differences in the plastic densities, e.g., due to temperature differences between hot nip rolls and cold pull rolls.
More specifically, the motors drive, or rotate, the active rolls at different speeds. For example, the first nip roll is driven at a surface speed slightly faster than the surface speed of the second nip roll to facilitate polishing one surface of the extruded sheet. The pull roll is driven at a surface speed even slightly slower than the second nip roll surface speed because the cooled plastic sheet has a higher density then the heated sheet drawn through the first and second nip rolls. The motor speeds typically are well regulated to facilitate rotating each roll at a desired speed.
Despite regulating the motor speeds, the active rolls for plastic sheet extruders sometimes experience speed fluctuations. These speed fluctuations typically are caused by gear transmission errors between the respective motors and rolls. For example, worm gear tooth profile errors, manufacturing errors, and deflections under load prevent a smooth transmission of power between the worm teeth and the worm threads in each two-stage worm gear transmission. As a result, there is a fluctuation in the normal and tangential forces at the transmission mesh, and the roll speeds, or surface speeds, fluctuate.
To reduce gear marks on plastic sheets, precise worm gearing in connection with the motors is sometimes used. However, worm gear tooth profile errors exist even with precise worm gearing. Therefore, although improving the precision of worm gearing may reduce the occurrence of gear marks, transmission errors still occur. These transmission errors also cause ripples on an extruded plastic sheet.
Other known apparatus for reducing gear marks include planetary gearing, fluid contact type transmission elements, and roller contact transmissions. These apparatus are directed at reducing, or eliminating, inherent transmission errors to reduce or eliminate ripples.
It would be desirable to improve the quality and reduce gear marks and other ripples in extruded plastic sheet. It also would be desirable to provide such improved quality even when transmission errors occur during extrusion.